Characterization of Rotational Stacking Layers in Large-Area MoSe2

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Characterization of Rotational Stacking Layers in Large-Area MoSe2 Film Grown by Molecular Beam Epitaxy and Interaction with Photon Yoon-Ho Choi,† Dong-Hyeok Lim,† Jae-Hun Jeong,† Dambi Park,† Kwang-Sik Jeong,† Minju Kim,† AeRan Song,‡ Hee-Suk Chung,§ Kwun-Bum Chung,‡ Yeonjin Yi,† and Mann-Ho Cho*,† †

Department of Physics, Yonsei University, Seoul 120-749, Korea Division of Physics and Semiconductor Science, Dongguk University, Seoul 100-715, Korea § Analytical Research Division, Korea Basic Science Institute, Jeonju, Jeollabuk-do 54907, Korea ‡

S Supporting Information *

ABSTRACT: Transition metal dichalcogenides (TMDCs) are promising nextgeneration materials for optoelectronic devices because, at subnanometer thicknesses, they have a transparency, flexibility, and band gap in the near-infrared to visible light range. In this study, we examined continuous, large-area MoSe2 film, grown by molecular beam epitaxy on an amorphous SiO2/Si substrate, which facilitated direct device fabrication without exfoliation. Spectroscopic measurements were implemented to verify the formation of a homogeneous MoSe2 film by performing mapping on the micrometer scale and measurements at multiple positions. The crystalline structure of the film showed hexagonal (2H) rotationally stacked layers. The local strain at the grain boundaries was mapped using a geometric phase analysis, which showed a higher strain for a 30° twist angle compared to a 13° angle. Furthermore, the photon−matter interaction for the rotational stacking structures was investigated as a function of the number of layers using spectroscopic ellipsometry. The optical band gap for the grown MoSe2 was in the near-infrared range, 1.24−1.39 eV. As the film thickness increased, the band gap energy decreased. The atomically controlled thin MoSe2 showed promise for application to nanoelectronics, photodetectors, light emitting diodes, and valleytronics. KEYWORDS: transition metal dichalcogenids, molybenum diselenides, molecular beam epitaxy, localized strain, rotational layer, optical band gap, dielectric dispersion

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INTRODUCTION Among the new semiconductor materials, graphene has attracted much attention because of its high mobility, transparency, and flexibility, resulting in its application to nanoelectronic devices. Recently, the application of graphene has been expanded to batteries, photodetectors, and electrodes. However, the zero band gap of graphene with a Dirac cone band structure is a major issue affecting its application to FET devices.1 Although studies addressing the opening of the band gap have concentrated on the use of methods such as a doping process and structural change, none have been successful. Unfortunately, the mobility is seriously degraded, even when the band gap is even slightly opened by the doping process and structural change. Recently, transition metal dichalcogenides (TMDCs) have attracted considerable attention because their large band gap (1−2 eV) is comparable to that of Si. In addition, they exhibit excellent and unique semiconductor characteristics such as high mobility, flexibility, and transparency. In particular, within the limitations of the monolayer, the band gap becomes a direct band gap (MoS2 = 1.9 eV, MoSe2 = 1.5 eV) from an indirect band gap because of the quantum confinement effect in the direction perpendicular to the layer plane.2 © 2017 American Chemical Society

In a layer with the chemical formula MX2 (M, transition metal for Mo and W; X, chalcogen for S, Se, and Te), a X−M− X covalent bond is formed, while adjacent layers are weakly held together by van der Waals (vdW) interaction.3 They exhibit structural polytypes such as trigonal prismatic coordination (2H and 3R) and octahedral coordination (1T).4−6 Although 1T TMDCs have metallic properties, 2H and 3R TMDCs have semiconducting properties and a band gap in the visible and near-infrared (NIR) range. Because of vdW coupling between the layers, one or a few layers of TMDCs can be easily prepared using a mechanical exfoliation method such as graphene preparation, in which the graphite is peeled off with tape. Such microexfoliated layers have been applied to various electronic devices such as field-effect transistors,7,8 photodetectors,9 light-emitting diodes,10 and catalysts.11 These applications have been demonstrated for a thickness of a few nanometers and a flake size on the order of micrometers. Because of the limits of the microsize and nonuniform flake Received: April 19, 2017 Accepted: August 15, 2017 Published: August 15, 2017 30786

DOI: 10.1021/acsami.7b05475 ACS Appl. Mater. Interfaces 2017, 9, 30786−30796

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ACS Applied Materials & Interfaces

the grown films with high coverage and crystallinity had twisted stacking structures containing localized strain around the grain boundary, the films had suitable band gaps, thus making them promising for application to optoelectronic devices.

thickness, the application of the exfoliation method has been limited to large-area device fabrication. To date, bottom-up approaches such as chemical vapor deposition (CVD), solution, and molecular beam epitaxy (MBE) methods have attracted researchers’ attention as a means of facilitating direct device fabrication without exfoliation. For example, a CVD system with a heating tube uses MoO3 and selenium powder as a precursor. A mixture of argon and hydrogen transports the gas phase elements, Mo and Se, to produce high-quality MoSe2.8,12 A spin-coating method has been reported as being a successful means of synthesizing a uniform film, the thickness of which is controllable. This method uses a reformulated DMF-based (NH4)2MoS4 solution with additional amine- and aminoalcohol-based solvents to enable the wafer-scale synthesis of uniform MoS2 thin films.13 In addition, the MBE method affords the growth of the highly crystalline MoSe2.14−17 In contrast to other growth methods, the MBE method was used in an ultrahigh vacuum environment, evaporating high-purity elements. This growth technique improves the control of the film thickness, film coverage, and uniformity. Synthesized large-area TMDCs have attracted interest for application to solar energy conversion. The reported data have shown that TMDCs can absorb 5−10% of the incident light, which is greater than that possible with GaAs and Si of the same thickness.18 Some data suggest that the absorption percentage for MoS2 can be increased up to 60% in the case of a film with a thickness of 30 nm. However, the defects generated during the growth process lead to device performance degradation. The grains of TMDCs are generated during the initial growth stage and are randomly distributed in the inplane direction. They expand in the lateral direction and eventually merge into a continuous film.19,20 During the growth process for multilayer structures, the layers are rotationally stacked with tilt angles because the interaction between the layers does not depend on chemical bonding but on weak vdW bonding. The rotationally stacked layers cause specific optical Moiré patterns depending on their tilt angles. In addition, the change in the interlayer interaction by the twisted structure tunes the electronic structures and phonon vibrational mode in the previous reports.21,22 Therefore, the lateral expansion of grains, and the interaction between the layers inevitably induce defects, such as vacancies,23 grain boundaries,24 and dislocations. Although defects can lead to the degradation of the optical device properties, very little research has addressed the effects of the physical characteristics caused by these defects. Herein, we describe the synthesis of large, homogeneous and multilayer MoSe2 films containing the defects, such as grain boundary and partial rotational stacking structure. The chemical composition and band diagram of the grown MoSe2 film were confirmed by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Photoluminescence (PL) measurements, as well as a mapping study, revealed the high quality of the film and homogeneous band structure around the K point in the Brillouin zone. In addition, scanning transmission electron microscopy (STEM) was conducted to verify the hexagonal structure (2H) of the film and rotational stacking layers in various locations. We determined the tilting angles from the diffraction pattern via the fast Fourier transform (FFT). On the basis of these angles, a geometric phase analysis was performed to determine the local strain field. Furthermore, the optical dispersion relationship and variation in the optical band gap with the film thickness were determined using spectroscopic ellipsometry (SE). Although

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METHODS

SiO2 with a thickness of 90 nm was thermally grown on highly doped p-type Si with an area of 2 × 2 cm2. The substrate was cleaned using acetone, methanol, and finally isopropyl alcohol. High-purity Se and Mo were simultaneously evaporated using a Knudsen cell and electron-beam evaporation, respectively. The evaporation rates of the selenium and molybdenum were 0.3 Å/s and 0.4 Å/min, respectively. The rate for the Mo source was controlled by monitoring the flux current. In order to minimize the impurities in the film, such as oxygen and carbon, the base pressure of the MBE chamber was maintained lower than 1.0 × 10−9 Torr. Under this condition, the impurities could be sufficiently extracted during the growth process. The working pressure in the MBE chamber was maintained at 8.0 × 10−9 Torr, and the substrate was annealed at 300 °C during the Mo and Se codeposition process. To improve the crystallinity of the MoSe2, a postannealing process was performed at 620 °C for 10 min. The areal diameter of the grown MoSe2, with a diameter of 1.9 cm, showed a higher coverage than that of TMDCs grown using the CVD growth method. The atomic composition and stoichiometry of the film were confirmed by examining the results of high-resolution XPS (PHI, VersaProbe). The Raman spectra were obtained using a micro-Raman spectrometer (Horiba Lab Ram ARAMIS) with a 532 nm laser and 1800 g/mm grating. The Si peak was fitted as 520.4 cm−1 to calibrate the data. The photoluminescence spectra were obtained using a LabRam HR PL (Horiba Jobin Yvon) with a 514 nm laser. The sample was transferred to a TEM grid using a solution of polystyrene (PS) dissolved in toluene. The film was coated with PS solvent and then baked for 15 min at 85 °C. Then, we scratched the polymer/MoSe2 at the edges before soaking the sample in water. The water thus penetrated the interface between the MoSe2 and SiO2 because of the difference in their surface energies. After several hours, the PS-coated MoSe2 films detached from the substrate and floated free in the water, with the TEM grid taking the floating polymer/ MoSe2. Finally, the grid was baked for 60 min at 80 °C to remove the water and then rinsed with a toluene solution to remove the polymer.25 Structural and atomic arrangement analyses of the synthesized MoSe2 films were performed using a JEOL ARM200F FEG-TEM/STEM with a hexapole corrector (CEOS GmbH) for the electron probe. Annular dark field (ADF) STEM analyses were performed with a probe current of approximately 20 pA, condenser aperture of 30 μm, camera focal length of 6 cm, and collection inner angle of approximately 90 mrad. The scanning rate of the ADF-STEM image was 6 μs/pixel for 512 × 512 pixels. All the STEM operations were performed at an accelerating voltage of 200 kV. A geometric phase analysis (GPA) script installed on a Gatan Digital Micrograph was used to calculate the strain generated at the grain boundaries between the rotationally stacked grains and the ordered stacked grains. Two sets of six spots were examined by applying the FFT to the STEM images containing the twisted layer. The nonlinear FFT components were chosen to define the lattice points for reference before the diffraction patterns were inverted into a phase image. The strain-mapping images were transformed after refining the g-vectors until the vectors had the same value. Spectroscopic ellipsometry (SE) was conducted using a rotating analyzer system with an auto retarder. The incident photon-waves were in an energy range of 435.0−1078 nm, for incident angles of 65°, 70°, and 75°.

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RESULTS AND DISCUSSION A MoSe2 film with a diameter of 1.9 cm was fabricated on the SiO2/Si substrate. We could easily use optical microscopy to observe the grown film, which appeared as a dark area, in 30787


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Figure 1. (a) Grown MoSe2 film appears as dark area, while SiO2/Si substrate is bright area through optical microscope. The 1.9 cm diameter film is shown in the inset in panel a. (b) Two typical lattice vibration modes for 2H MoSe2 are observed at 240.06 and 289.87 cm−1, which correspond to the A1g and E12g modes, respectively. The spectra are consistent at three points, namely, the center, middle, and edge of the film. (c) The PL spectra were measured on the MoSe2 with three layers. The two resonant features, A and B excitons, are observed at 1.57 and 1.77 eV, respectively. The PL spectra for three points across the film show no significant change. (d) In the mapping measurements, the intensity for the A exciton demonstrates the homogeneity of the MoSe2 film within a 100 × 100 μm2 area. The peak position corresponding to the intensity is shown in the inset of panel d.

conduction band minimum at the midpoint between the Γ (ΓCBM) and K positions (KCBM). A reduction in the number of layers causes a transition from the indirect band gap to the direct band gap (KVBM to KCBM), which is induced by the quantum confinement effect in the perpendicular direction.2 Although the film has an indirect bandgap, the clear spectral features caused by the two recombination processes are shown in Figure 1c. The two emissions, A and B excitons, are a result of the direct transition at the K point in the Brillouin zone, and correspond to emission energies of approximately 1.57 and 1.77 eV at room temperature, respectively.14,28 The energy difference between the A and B peaks (approximately 200 meV) corresponds to the value of the valence band splitting caused by the strong spin−orbit coupling of the heavy atoms with a large angular momentum.29,30 It is very important that the states caused by the excitons are generated even at room temperature, because valleytronics using the exciton states could be applied to devices based on a new concept. The long-range uniformity of the film could also be confirmed from the emission observed over large areas of the film surface. No significant changes were observed in the emission characteristics, regardless of the position on the film. In addition, within the microscopic area (100 μm × 100 μm), the mapping images, according to the maximum PL intensity and corresponding peak, are shown in Figure 1d and its inset, respectively, demonstrating the excellent homogeneity of the MoSe2 film. To observe the growth stage for the atomically thin and homogeneous film, the STEM measurements were conducted with growth-time. The STEM image shows the contrast difference with the atomic number. The STEM images with low magnification in the Figure 2 indicate that the areas marked

contrast to the substrate, which appeared as a bright area, as seen in Figure 1a. Considering reports of atomically controlled MoSe2 being grown on a crystalline substrate to minimize the lattice mismatch, it is interesting to note that large areal MoSe2 layers were grown on the amorphous substrate in this study. Atomic force microscopy (AFM) measurements showed that the film had a smooth surface with a root-mean-square (RMS) value of 0.288 nm and thickness of 1.96 nm, which corresponded to the three layers, as shown in Figure S1a and b. To confirm that the film was homogeneous, spectroscopic measurements were performed across the film. The Raman active modes for the synthesized 2H-MoSe2 showed two distinct features: the A1g mode for out-of-plane vibration and E12g mode for in-plane vibration. In Figure 1b, the A1g and E12g peak positions are located at 240.06 and 289.87 cm−1, respectively.12 The lattice-vibrational modes represent the 2H-phase for the grown MoSe2.26 The full width at halfmaximum (fwhm) of the A1g is 3.87 cm−1, indicating the presence of a good crystalline structure.27 Furthermore, numerous Raman measurements were also examined over a wide area, as shown in Figure 1b. Each spectrum is in good agreement with the typical Raman mode for MoSe2, and the fwhm values at the center, middle, and edge positions are almost the same. Thus, we can confirm that the film was homogeneously synthesized with a single phase, 2H, regardless of the position within the sample. In addition, PL measurements at room temperature were conducted on the MoSe2 film with three layers to observe the optical band gap and band structure caused by the direct transition. In the bulk characteristics, the TMDCs have an indirect band gap from the valence band maximum at the Γ position (ΓVBM) to the 30788


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Figure 2. STEM image was conducted to show the initial growth stage. The arrows indicated in a similar color, A, B, and C are monolayer, bilayer, and empty space, respectively. (a) The dark and bright areas represent a continuous monolayer and partial bilayers on the film, respectively. (b) With the deposition time increased, the bilayer expands in the lateral direction. (c) The rotation of the stacking layer by θ° is shown in the cross-sectional view. (d) The rotational stacking layer is partially observed on the continuous monolayer because the interaction between the adjacent layers is held by the vdW force.

by the arrows A, B, and C represent a monolayer, bilayer, and empty space, respectively. The STEM image, which shows a monolayer in the dark area and bilayer in the bright area, clearly shows that the layered growth can be achieved in Figure 2a, that is, a continuous MoSe2 monolayer and partial bilayer on the monolayer were observed. The process of nucleation on the amorphous substrate can be explained by the relation with the Gibbs free energy (ΔGr,γ,) and nucleation rate (NT). ΔGr,γ is given by −VnΔGV + ∑kγkAk, where the left term refers to the volume free energy and the right term implies the surface energy. The Vn, ΔGV, γ, and A terms represent the volume of the nucleation, difference in the Gibbs free energy per unit volume, surface energy, and area of the nucleation, respectively. Considering the shape of the nucleation to be a cylinder, the left and right terms can be expressed as − πr2tΔGV and πr2(γc + γsc − γs) + 2πr γc,edge, respectively, where r, t, γs, γc, γsc, and γc,edge represent the radius and height of the nucleation, surface energy of the substrate, nucleation of MoSe2, interface between the MoSe2 and substrate, and nucleation of MoSe2 at side of the nucleation, respectively.31 The energy barrier to nucleation, ΔG*, is determined at the critical radius, r*, which is under the condition of dΔG*SiO2/dr = 0. Using ∼310 mJ/m2 for the surface energy of SiO2 (γs) and ∼75 mJ/m2 for the surface energy of MoSe2 (γc), the energy barrier for the formation of nucleation * 2,32,33 is higher than that of the nucleation on on SiO2, ΔGSiO MoSe2, ΔG*MoSe2. Even if the ΔG*SiO2 has a larger energy than * 2, the nucleation on the SiO2 can be generated that of ΔGMoSe

using supersaturation as a driving force. Here, ΔGV is

ΔHV ΔT , Tm

where ΔHV and ΔT represent the required heat of the phase change and temperature difference between the grow-temperature (Tgrowth) and melting temperature (Tm), respectively. ⎛ ΔG* ⎞ E Then, NT can be expressed by exp − k Td × exp⎜ − k T(T) ⎟, ⎝ B ⎠ B where Ed is the activation energy for diffusion. The left term represents the frequency at which thermally activated atoms attach to the nucleus. The right term means the probability distribution of nucleation. At a high temperature below the Tm, the ΔG* has a higher energy than that of Ed. This means the nucleation is suppressed because the diffusion probability is high and nucleation probability is too low. Because the ΔG* is lower energy than that of Ed, in decreasing the temperature, the ⎛ ΔG* ⎞ E exp⎜ − k T(T) ⎟ has a larger value than exp − k Td . That is, the ⎝ B ⎠ B nucleation increases with the temperature decrease. At a low

( )

( )

( ) E

temperature, because the exp − k Td is too small, resulting in a B

decrease in the nucleation rate (NT). Therefore, NT is maximized at a certain Tgrowth below the melting temperature. Because the Tgrowth is dependent on the Gibbs free energy. The nucleation can be increased on the amorphous SiO2 substrate at a specific growth temperature, compared with that of MoSe2. In other words, the nucleation on SiO2 prefers to expand in the lateral direction rather than the vertical direction. Moreover, 30789


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Figure 3. Atomic images are measured by TEM and STEM. The TEM image is shown in panel a, and STEM images are shown in panels b, c, and f. (a) The MoSe2 was transferred on TEM grid. The dark region represents the folded film, which is indicated by the yellow-dotted line. (b) The MoSe2 has a honeycomb atomic arrangement which is a hexagonal structure, showing a good crystallinity. (c) The yellow and blue circles indicate the Se2/Mo/Se2 stacked layers and Mo/Se2/Mo stacked layers, respectively. (d) The intensity of the atomic brightness along the dotted line in (c) distinguishes the stacking sequence types. The higher and lower intensities represent the order of Se2/Mo/Se2 and Mo/Se2/Mo, respectively. (e) Diffraction by FFT reveals a set of six spots. (f) Cross-sectional ABF-STEM image showing three layers. The first, second, and third black lines indicate a MoSe2 film layer, indicating a layered film structure.

transfer the film on the TEM grid, although some folded parts could not be completely controlled, as shown in the region indicated with the arrows in Figure 3a. The structure of the film in the folded area was also investigated in detail (see Figure S1c). The most stable and dominant structure among the three crystal polytypes of MoSe2 (1T, 3R, and 2H) was the hexagonal structure (2H), which had semiconductor characteristics.4,6 In Figure 3b, the annular dark field (ADF) image, which used an HRTEM filter script, reveals a 2H structure representing the honeycomb arrangement of the atoms. Because the intensity of the lattice points is related to the atomic number, we could confirm the atomic arrangement of the film. For the three-layer film, the atoms could be stacked in either of two configurations, Se2/Mo/Se2 or Mo/Se2/Mo, as shown in Figure S3. (A2/a/A2 indicates that A2, a, and A2 are located in the top layer, middle layer, and bottom layer, respectively.) The line profile shown in Figure 3c reveals a clear intensity difference between two points represented by the yellow and blue circles. Considering the intensity change with the atomic number, the yellow and blue circles can be assumed to correspond to Mo/Se2/Mo and Se2/Mo/Se2 arrangements, respectively. The distance between the lattice points for one side of the honeycomb is 1.93 Å, which corresponds to the distance between the green and blue triangles in Figure 3c, and for the long diagonal line, 3.67 Å, which is the length between the blue and red triangles, shown in Figure 3d. This is consistent with the atomic structure of MoSe2. The FFT of the high-crystalline 2H MoSe2 structure with only a few layers shows typical diffraction spots with the set of 6-fold symmetry, indicating that the hexagonal structure has the reported honeycomb shape, as shown in Figure 3e. From the diffraction patterns, we can extract the distance between various planes, for example, the spacing between the (10−10) planes is 2.74 Å, and that between the (11−20) planes is 1.58 Å. Moreover, to determine the exact numbers of layers and their thicknesses, a microscopic measurement of the cross-sectional plane was conducted, as shown by the Annular Bright Filed (ABF)-STEM

because the atoms can be easily attached to the kink sites at the boundary of the nuclei to minimize the surface energy, growth in the lateral direction can be further promoted. The expanding grains with lateral direction merge to form a continuous film with a grain boundary, such as a mirror twin boundary (MTB).20 The growth mechanism involves layer by layer growth, expanding the bilayer gradually on the covered monolayer with the deposition time, as shown in Figure 2b. In addition, the two grains in the different layer can be smoothly merged, generating the structural deformation in the boundary in the Figure S2c. However, in this growth process, the different layer number generally can exists in the film with atomically smooth roughness. Therefore, the grown film is predominantly trilayer and partially bilayer in Figure S2. It remains a challenge to grow centimeter-scale TMDCs having only a single layer. In layered materials, such as graphene, h-BN, and TMDCs, the twisted layer was observed because the interaction between the adjacent layers is limited by a physical force (vdW force) without any chemical binding.34−36 In this case, Figures 2c and d show the atomic simulation of the rotational stacking and the STEM image with the twisted layer, respectively. The tuning of the rotational stacking-disorder is a critical issue because it can modify the electronic and optical properties. The method for controlling the angles during growth needs to be studied further. To evaluate the atomic arrangement of the MoSe2 films in detail, STEM measurements at high magnification were performed for in-plane and cross-sectional characterization. The preparation of the samples for the high resolution-STEM measurements was critical because wrinkling, folding, and polymer residues in the transfer process make it difficult to measure the undistorted atomic arrangement. To overcome these drawbacks, we used a specific delamination process based on the difference in the surface energies of MoSe2 and SiO2, that is, the SiO2 on p+Si is hydrophilic, whereas the grown MoSe2 film is hydrophobic. With the bright-field TEM measurement, the process was confirmed to successfully 30790


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Figure 4. (a and e) Twisted staking layer is shown because the interaction between adjacent layers are hold by vdW force. (b and f) The twist angles were confirmed by measuring the angle between the oriented and twisted diffraction patterns. The rotational angles are the 13° and 30°, respectively. (c and g) The atomic configuration for the rotationally stacked layers is simulated. The green and blue circles denote selenium and molybdenum, respectively. The faint color indicates the below layer elements. (d and h) The GPA program was applied to induce a strain in the film. The strain (εxy) mapping overlapped on the ADF-STEM images containing the stacked layers twisted by 13° and 30°.

structure can be verified by the different angles of two sets between the yellow spots and red spots. The angles obtained from the Moiré patterns caused by the change in the stacking order were measured and found to be 13° and 30° in Figure 4b and f, respectively. We simulated the rotationally aligned stacking order for the three-layer film. From the STEM images, it is difficult to discriminate which layer is rotated. However, the typical Moiré pattern shapes can be obtained because the shapes are all the same regardless of the order of the twisted layers. The atomic configuration of the trilayer MoSe2, consisting of layers that are twisted by 13° and 30°, are shown in Figure 4c and g, respectively, in which the atoms colored green and blue are the Se and Mo atoms, and the fainter color denotes below layer elements, respectively. The simulated atomic arrangement is in good agreement with the observed STEM images shown in Figure S4. The formation of the rotational stacking structure could induce a local strain field on the layer, which could also modulate the electronic structure in the local region. In general, the change in the electronic structure caused by the local strain field could be induced around the grain boundary between oriented and mis-oriented stacking layers. To investigate the strain field in the grain boundary, we used a GPA tool, incorporated into the Gatan Digital Micrography program.40 STEM images containing Moiré patterns were converted into two sets of six diffraction spots by FFT. A phase image was extracted by choosing two nonlinear spots in the diffraction pattern. Finally, the strain components were extracted using the phase and reciprocal lattice vectors. The local strain factors can be visualized through the mapping process with εxx (∂ux/∂x), εyy (∂uy/∂y), and εxy (∂ux/∂y) applied to the grain boundary. Figure 4d and h show the overlapped strain mapping for εxy with the raw atomic images containing 13° and 30° twisted grains, respectively. (For the εxx, εyy mapping, see Figure S5.) The mapping data clearly show that strain is induced around the grain boundary, that is, shear strains of approximately ±7−8% at the lower tilted grain boundary and approximately ±10−14% at the higher tilted grain boundary are induced. Considering the atomic picture of

image in Figure 3f. The three layers can be clearly distinguished from the dark lines in the ABF-STEM image. The STEM inplane images show that the crystalline quality and atomic arrangement of a film grown on amorphous SiO2 are comparable to those of MoSe2 grown on a crystalline substrate by the MBE method. While the literature states that MoSe2 film growth occurs only on a crystalline structure based on a lattice match,15,16,37 we successfully grew a layered MoSe2 film with a controlled thickness and a high crystalline structure on an amorphous layer. Although MoSe2 with a two-dimensional layered structure could be effectively grown, layers rotated relative to the other layers and the grain boundaries were observed in the local region. The Moiré patterns caused by the formation of defects during the film growth were unavoidable because only a vdW force was acted between the stacked layers, and nucleation was expanded in lateral direction to form a continuous film. Although the reported TMDC films also contained rotational stacking disorders and grain boundaries, detailed analyses of the atomic arrangement in a plane view and the defects on the layer are needed. Figure 4a and e clearly show the rotational stacking disorder, as shown in the formation of the two characteristic types of Moiré patterns. The rotational stacking structure can be attributed to the formation energy and interlayer interaction. The difference in the formation energy between 2H and 3R is about 1 meV/ Å. In addition, the interlayer binding energy is 20 meV/Å.38,39 This causes a rotational stacking configuration in the layered compound materials. Because our growth film has a single phase of 2H, the rotational stacking disorder can be increased due to the minimization of the total MoSe2 system energy. The diffraction patterns seen in the real image through the FFT process clearly show two sets of typical shapes, each containing six spots, as shown in Figure 4b and f. The reciprocal lattice points represented by the yellow and blue spots are caused by the 2H-MoSe2 structure with an AB stacking structure, for which θ = 60°, while those indicated by the red and green spots result from the twisted stacking-order structure. The tilted angles in the twisted stacking-order 30791


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Figure 5. XPS measurements were conducted to confirm the chemical composition and stoichiometry of film. (a and b) The spectra correspond to each elemental core level, that is, the 3d orbital. The spectra of Mo4+ and Se2− show the binding energy of MoSe2. (c) The UPS spectra indicate that the valence band maximum and work function were estimated to be 1.10 and 4.64 eV, respectively. (d) The energy band diagram for the MoSe2 grown by MBE was plotted, demonstrating the n-type semiconductor.

regardless of the point position. The slight ratio difference between the measured and typical values can be attributed to defects formed in the sample, such as dislocations, vacancies, and grain boundaries between rotationally tilted stacked layers and oriented stacked layers.19,20 With the estimated stoichiometry, the Se-deficiency can generate an MTB with a 60° symmetry in a continuous film.20,23 However, it is difficult to observe the MTB by STEM measurement due to the overlapping atomic images of the other layers. Ultraviolet photoemission spectroscopy (UPS) was conducted to evaluate the work function (ϕ) and valence band (VB) for the synthesized MoSe2 film. The VB of 1.10 eV for the film was determined using a linear method, as shown in Figure 5c. Because the point and area defects generate localized states in the band gap,19,20 the orange area in the spectrum originates from defects, such as the grain boundaries generated by the grains expanding in the plane direction and the rotational stacking order resulting from the weak vdW interaction between layers. The inset of Figure 5c shows the secondary electron cutoff (SEC) region as a kinetic energy scale. The work function is easily determined to be 4.64 eV. The band alignment with the optical band gap, which is lower than that of the real band gap, VB, and ϕ indicate that the grown MoSe2 has n-type semiconductor characteristics, as shown in Figure 5d.8 The interaction between the thin film and photons was studied by Raman and SE measurements to investigate the effects of the rotational stacking disorder. As the thickness decreased, the restoring force was reduced by the weaker interlayer coupling, following a change in the atomic interaction in the in-plane direction.43 The report of the MoS2 surface structure showed that the distance between Mo and S decreases up to 5% at the topmost layer, suggesting a surface

the tilted grain, the strain in the 30° tilted grain boundary may be higher than that in the 13° tilted grain boundary in the inplane direction. Moreover, the highest extent of the strain would be induced at the 30° tilted grain considering the D3h crystal structure of the MoSe2. It is interesting that the control of rotationally stacking angles can tune not only the strain in the film but also the electronic structure.20,41,42 Although the GPA results do not clearly show the change in the strain within the Moiré pattern area away from the grain boundary, the induction of the strain at the grain boundary can be clearly observed. Another possible change in the local strain is generated in the vertical direction: the distance between the intralayers can be modulated depending on the twist angles.42 In order to evaluate the chemical composition and elemental ratio of the film containing the rotational stacking layers and localized strain around the grain boundary, X-ray photoemission spectroscopy (XPS) measurements were performed. The binding energy of carbon, 284.8 eV, was used as a reference peak position. The measured 3d orbitals of the Mo and Se core levels in the MoSe2 sample are shown in Figure 5a and b.12 In Figure 5a, the Mo4+ peaks are located near 229.27 and 232.41 eV, which correspond to the 3d5/2 and 3d3/2 orbitals, respectively. The peaks of the Se 3d orbitals around 54.82 and 55.67 eV correspond to the 3d5/2 and 3d3/2 orbitals, respectively, as shown in Figure 5b. The relative element ratio is estimated by the spectral areas for Mo 3d and Se 2d. The stoichiometric ratio is 1:1.96. In addition, the difference in the binding energy of Mo 3d5/2 and Se5/2, 174.45 eV, indicates that the elemental chemical bounding is consistent with that of MoSe2. Numerous XPS measurements were conducted to assess the stoichiometry across the sample (see Figure S7), which indicated that the MoSe2 films have the 1:2 ratio, 30792


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Figure 6. Optical interaction with MoSe2 are observed with the range from 3 to 8 layers. (a) Raman spectra showed the shift of two vibration modes. (b and c) The optical dispersion of MoSe2 with the number of layers verified by SE measurement. The real and imaginary parts of dielectric dispersion, ε(v) = ε1(v) + iε2(v), are plotted. The dielectric value in (b) can be affected by the rotational stacking disorder and grain boundaries. (c) The imaginary part, ε2, indicates the transition of the band to band. The two resonance features in (c) are observed around 1.50 and 1.69 eV. (d) The tauc’s plot of the MoSe2 was calculated by α(hv) = K(hv − Eg)m/hv for m = 2. The optical band gap extrapolated in the linear portion was shown in the inset of panel d.

reconstruction. This change can accommodate the lattice vibration in the intralayer. Thus, in Figure 6a, a change in the Raman mode caused by interlayer coupling and surface reconstruction is observed: the A1g peak becomes red-shifted 1 peak becomes blue-shifted as the thickness and E 2g decreases.28,44 The shift in the Raman active mode with the thickness is plotted in Figure S7, and the values of the Raman shift are summarized in Table S1. It is interesting that the change in the Raman shift as a result of the in-plane vibration, E12g, is more noticeable than the change in the A1g mode, because the in-plane vibration mode is sensitive to the structural deformation.45−47 The most plausible structural deformation in the thickness of a uniform film with a large area is the strain induced around the grain boundary. Furthermore, the difference in the thermal expansion coefficient between the SiO2 and MoSe2 can affect the strain during the annealing and cooling processes. In addition, the SE data showed that the optical dispersion of the grown MoSe2 was dependent on the number of layers. The dielectric functions with the frequency, ε (ν) = ε1 (ν) + iε2 (ν), were obtained for the film with different layers, as shown in Figure 6b and c. Two resonance features around 1.50 and 1.69 eV are observed in the spectrometry of Figure 6b, which are comparable tendency to those in the case of MoS2, which has two resonance peaks at 1.9 and 2.05 eV.48 The dielectric value of the grown film, 13−17, was obtained in this study, whereas those of the exfoliated MoSe2, 20 for the monolayer and 30 for the bulk, were reported. The dielectric relationship can be affected by the doped charge, strain, and nonhomogeneity. Because there was no difference between the doped charge and

nonhomogeneity, as shown in the structural data, the slight difference between the deposited film and exfoliated flakes can be attributed to the rotational stacking disorder and grain boundaries. The localized strain and increase in the interlayer spacing caused by the rotational stacking disorder could induce variations in the electronic structure, charge distribution, and interlayer coupling.20,35,49 Figure 6c shows the change in the dielectric function in the imaginary part with the number of layers. The typical peak positions at 1.50 and 1.69 eV are associated with the direct transition at the K point in the Brillouin zone, which is the same origin as that observed in the PL excitation characteristics. The peak position difference between A (the lower photon energy) and B (the higher photon energy) is attributed to the valence band splitting at 0.19 eV, which is in good agreement with the valence band splitting energy observed in the PL measurement results described above. We extracted the approximate optical band gap using a tauc’s plot, α(hν) = K (hν − Eg)m/hν, where K is a constant, hν is the incident photon energy, and m is 1/2 for direct transition and 2 for indirect transition.50 The optical coefficient (α) is given by 4πk/λ, where k is the extinction coefficient and λ is the wavelength. The extinction coefficient with photon energy is shown in Figure S8. The optical absorption was extracted by substituting 2 for m because MoSe2 grown with more than three layers is regarded as having an indirect band gap.51 Figure 6d shows the change in the optical absorption as a function of the film thickness using the spectra of the extinction coefficient. The optical band gap, Eopt g , was extrapolated in the linear portion around the onset of the absorption edge to (αhv)1/2. In the inset of Figure 6d, the 30793


Research Article

ACS Applied Materials & Interfaces variation in Eopt as a function of the number of layers is g confirmed.3 Thus, Eopt = 1.39 eV for three layers, gradually g decreases to Eopt g = 1.24 eV for eight layers, corresponding to the change in the electronic band gap.3 The range of the optical band gap, 1.24−1.39 eV (for which the wavelength ranges from 1000 to 892 nm), corresponds to the NIR energy scope. The range of the optical band gap is such that MoSe2 is a promising candidate material for application to optoelectronic devices. In addition, we fabricated typical photodetector devices to investigate the practicality of the material’s application to optoelectronic devices. The devices were patterned using electron-beam lithography, with a channel length of 3 μm. The channel was focused using a 532 nm laser with a beam size of 2 μm. The ON state current under illumination was enhanced by a factor of 32 relative to the OFF current (Figure S9) The photocurrent could be further improved by controlling the grain size and interface between the MoSe2 and SiO2/Si.

increasing the ON current. Because MoSe2 with high coverage and crystallinity, as well as a suitable band gap range, can be successfully synthesized using an MBE system, a film that is uniformly fabricated by a vacuum process can be more conveniently applied to the fabrication of a device with a large area without the need for any transfer process.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05475. Two Raman peaks, A 1g mode and E 1 2g mode, summarized according to number of layers; AFM measurement examining surface roughness and thickness of the film; high-resolution TEM image showing layered structure of the MoSe2 film at the folded edge, indicating the three layers; atomic arrangement of 2H structure showing honeycomb structure in plane and vertical view; supercells with stacking layers rotated by 13° and 30°; strain mapping for εxx and εyy overlapped with STEM images containing stacked layers rotated by 13° and 30° relative to bottom layer; variations in Raman shifts are plotted with the thickness of film; extinction coefficient (k) plotted with 3-layer MoSe2 film; and photodetector device was illuminated with 532 nm laser (PDF)

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CONCLUSION We succeeded in synthesizing long-range uniform MoSe2 over a large area by the application of a MBE growth method. The XPS and UPS measurements verified the chemical composition and element ratio of the grown film, indicating that the film was an n-type semiconductor. The Raman and PL spectra indicated the representative phonon vibration modes and optical band gap for the layers, respectively. Furthermore, by confirming the long-range uniformity of the MoSe2 film by conducting mapping and measurements across a wide area, we found that the growth method was suitable for the synthesis of TMDC for large, homogeneous films. The STEM images showed a highly crystalline structure and typical Moiré patterns in part of the film. The Moiré pattern caused by the rotational stacking structure was surrounded by a grain boundary. Because the weak interaction between layers induced a rotational stacking structure, the structural characteristics should be generated during the layered film growth. The growth method to control the twisted angles requires further study, resulting in the band gap modulation. The developed growth method can be appropriate for the two-dimensional materials with a strong interlayer interaction, such as PtS2 and PtSe2.52,53 With the GPA program, we determined that the distribution of the strain in the in-plane direction was localized around the grain boundary in the film. The strain near the grain boundary formed by the 30° twisted layer was higher than that formed by the 13° rotated layer. The localized strain could lead to a change in the electronic structure and the generation of defect states in the band gap. Because with additional doping fabrication, the substituted dopant can be easily located at the defect sites, the localized strain could be modulated and the electronic structure could be controlled. In addition, the optical dispersion relationship of the film containing the localized strain was confirmed by SE measurements. The dielectric relationship with photons was represented by two typical resonance peaks at 1.50 and 1.69 eV, corresponding to the direct transition at the K point in the Brillouin zone. A slight difference in the dielectric dispersions between the deposited film and exfoliated flakes could have been generated by the rotationally stacked structure with grain boundaries in the film. The optical band gap was within the NIR range, which showed the possibility of applying the film to optoelectronic devices. Finally, we fabricated photodetector devices that exhibited an enhanced ON current. Controlling the grain size will be the subject of future research, with the aim of

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yeonjin Yi: 0000-0003-4944-8319 Mann-Ho Cho: 0000-0002-5621-3676 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge the financial support from National Research Foundation of Korea (NRF) (Grant No. 2017R1A5A1014862, SRC program vdWMRC center). This work was supported by a NRF grant funded by the Korea government (MSIP) (No. 2015R1A2A1A01007560) and by an Industry-Academy joint research program between Samsung Electronics and Yonsei University. The authors thank Dr. HeeSuk Chung of the Korea Basic Science Institute at Jeonju for technical assistance in the TEM and STEM measurements.

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REFERENCES

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Characterization of Rotational Stacking Layers in Large-Area MoSe2

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